It is well known that to optimize signal transmission to and/or from a circuit element and a signal line to which it is connected, the impedance of the circuit element, i.e., its termination impedance, should match as closely as possible the impedance of the signal line. This can be difficult, since integrated circuit technology has inherent variability in process parameters that affect the impedance of circuit elements. Thus, the same circuit element may have different impedances from IC to IC, simply because of this variation in process parameters.
Numerous schemes have been utilized to overcome this problem, with varying degrees of success. However, such schemes can be limited in their accuracy, and have a limited range over which they may be tuned, if, indeed, they can be tuned at all. With modern trends in electronics driving not only circuit element size ever smaller, but also signal levels, it is becoming even more critical to be able to match termination impedances with signal lines with high accuracy, and over a wide range. In fact, it would be desirable to not only provide termination impedances that are adjustable to compensate for process parameter variations, but also to provide continuous calibration of termination impedance to compensate for variations arising from environmental factors such as temperature.